1. Field of the Invention
This invention relates to a temperature sensor circuit and a constant-current circuit, and more particularly, to a temperature sensor circuit and a constant-current circuit which are formed of MOS transistors on a MOS-type integrated circuit.
2. Description of the Related Art
A conventional temperature sensor circuit is formed of bipolar transistors in general as shown in FIGS. 1 and 2. The circuits shown in FIGS. 1 and 2 each comprises n transistors Q1, Q2, . . . , Qn of either npn- or pnp-type, n constant-current sources for supplying n constant currents I1, I2, . . . , In respectively to the n transistors Q1, Q2, . . . , Qn, and a constant-voltage source for applying a constant voltage Vcc to each of the n transistors Q1, Q2, . . . , Qn. The operation of these circuits will be explained below.
In FIG. 1 or 2, supposing that a current and saturation current of each transistor Qj (j=1, 2, . . . , n-1, n) are expressed as Ij and Is, respectively, the Boltzmann constant is expressed as k, absolute temperature is expressed as T, the unit electronic charge is expressed as q and a base-to-emitter voltage of the transistor Qj is expressed as VBEj, the current Ij (j=1, 2, . . . , n-1, n) of each of the n constant-current sources can be shown as follows; ##EQU1##
Here, supposing that the direct current amplification factor of the transistor Qj is sufficiently high and the base current thereof is negligible, the base-to-emitter voltage VBEj of the transistor Qj can be shown as follows; ##EQU2##
As a result, in the circuit shown in FIG. 1, an output voltage Vo of the circuit can be shown as follows; ##EQU3## On the other hand, in the circuit shown in FIG. 2, the difference (Vcc-Vo) between the voltage Vcc of the constant-voltage source and the output voltage Vo of the circuit can be shown as follows; ##EQU4##
As seen from Eqs. (3) and (4), in the temperature sensor circuit shown in either FIG. 1 or 2, the voltage proportional to absolute temperature T can be obtained. This means that the temperature at a position where the temperature sensor circuit is disposed can be detected.
However, if such a conventional temperature sensor circuit as expressed above is to be realized on an MOS-type integrated circuit by using MOS transistors instead of bipolar transistors, it is difficult to obtain a sufficiently large direct current amplification factor due to the fact that the MOS integrated circuit uses parasitic bipolar transistors, so that the actual output voltage Vo is largely deviated from a value calculated from Eq. (3) or (4), thus arising such a problem that it is disadvantageous to be practically used.
In addition, in case of receiving an output of the temperature sensor circuit by, for example, an A/D converter, since the above-mentioned conventional temperature sensor circuit is not of a differential output type, a reference voltage source is disadvantageously necessary to be used additionally. If the reference voltage source is used additionally, the temperature characteristic that it has may degrade the characteristics of the temperature sensor circuit itself.
Next, as a constant-current circuit formed of MOS transistors, a circuit as shown in either FIG. 3 or 4 is well-known conventionally.
The constant-current circuit shown in FIG. 3 is one that was disclosed in "IEEE Journal of Solid-State Circuits", Vol. SC-12, No. 3 June 1977, pp. 224 to 231, in which the MOS transistors are operated under the weak inversion state thereby to obtain an output current IR as small as 30 nA to 2 .mu.A.
On the other hand, the circuit as shown in FIG. 4 is disclosed in "VLSI Design Techniques for Analog and Digital Circuits", pp. 363 published by McGraw-Hill, 1990, which is a called boost strapped current source/sink. In this circuit, a drain current ID61 of a MOS transistor M61 and a drain current ID62 of a MOS transistor M62 are related by the following equations (5) and (6), where VTH is a threshold voltage, KN' is a conductance, L61 is a gate length and W61 is a gate width. EQU ID61=ID62 (5) EQU ID62=(VTH/R)+(1/R).multidot.(2 ID61.multidot.L61/KN'.multidot.W61).sup.1/2( 6)
With the constant-current circuit shown in FIG. 3, the output current is as small as 10 nA to several micro-amperes (.mu.A), resulting in arising such a problem that a practical operational level of current (about several hundreds micro-amperes) cannot be obtained unless otherwise sophisticated more.
With the constant-current circuit shown in FIG. 4, the threshold voltage VTH is varied, so that when taking notice of the manufacturing deviation, the dispersion of the output current is excessively large.
Thus, an object of this invention is to provide a temperature sensor circuit of a differential output type formed of MOS transistors.
Another object of this invention is to provide a temperature sensor circuit of a differential output type formed of MOS transistors on a CMOS integrated circuit.
Further another object of this invention is to provide a constant-current circuit which is capable of obtaining a current at a value of practical operational level as well as setting the current with no effect by variation of a threshold voltage, thus being adapted to be used for a CMOS integrated circuit.